Date: Tue, 3 Oct 2017 11:04:33 +0100 From: Mark Rutland <mark.rutland@....com> To: Kees Cook <keescook@...omium.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Michal Marek <mmarek@...e.com>, Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, Ingo Molnar <mingo@...nel.org>, "David S. Miller" <davem@...emloft.net>, Nicholas Piggin <npiggin@...il.com>, Al Viro <viro@...iv.linux.org.uk>, Laura Abbott <labbott@...hat.com>, linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH 0/3] Makefile: Introduce CONFIG_CC_STACKPROTECTOR_AUTO Hi Kees, On Mon, Oct 02, 2017 at 12:20:04PM -0700, Kees Cook wrote: > As described in the final patch: > > Nearly all modern compilers support a stack-protector option, and nearly > all modern distributions enable the kernel stack-protector, so enabling > this by default in kernel builds would make sense. However, Kconfig does > not have knowledge of available compiler features, so it isn't safe to > force on, as this would unconditionally break builds for the compilers > or architectures that don't have support. Instead, this introduces a new > option, CONFIG_CC_STACKPROTECTOR_AUTO, which attempts to discover the best > possible stack-protector available, and will allow builds to proceed even > if the compiler doesn't support any stack-protector. > > This option is made the default so that kernels built with modern > compilers will be protected-by-default against stack buffer overflows, > avoiding things like the recent BlueBorne attack. Selection of a specific > stack-protector option remains available, including disabling it. I gave this a spin atop of v4.14-rc3 with a few arm64 toolchains I had installed: * Linaro 17.08 GCC 7.1 // strong * Linaro 17.05 GCC 6.1 // strong * Linaro 15.08 GCC 5.1 // strong * Linaro 14.09 GCC 4.9 // strong * Linaro 13.06 GCC 4.8 // none * Linaro 13.01 GCC 4.7 // none AFAICT, the detection is correct, and arm64 toolchains only gained stack protector support in GCC 4.9. I manually tested GCC 4.8 and 4.7, and got: warning: -fstack-protector not supported for this target [enabled by default] ... so that looks good to me. One thing I noticed was taht even when the build system detects no support for stack-protector, it still passes -DCONFIG_CC_STACKPROTECTOR to the toolchain. Is that expected? Thanks, Mark.
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